Light fixtures that use light emitting diode (LED) technology for illumination are gaining in popularity. These fixtures are now employed more frequently in commercial, residential and public settings. The main reasons that LED-based light fixtures are becoming more popular are that they generally have a longer operational life and operate at a much higher power efficiency. For example, LED-based light fixtures typically have an operational life of around 50 to 100 thousand hours; whereas, incandescent-based light fixtures typically have an operational life of only one to two thousand hours. Additionally, LED-based light fixtures typically have a light efficacy that is 5 to 10 times that of an incandescent light fixture.
Driving or supplying power to LED-based light fixtures, however, may need more consideration to ensure substantially constant illumination. In the past, LED-based light fixtures have been driven by constant output voltage and constant output current ballasts. However, these devices generally do not provide constant power to LED-based loads, and thus, cannot ensure constant illumination of the luminous loads.
Taking, as an example, a constant output voltage ballast, it employs output voltage feedback to ensure that the voltage across an LED-based load is substantially constant. However, the junction voltage of LED devices decreases as environment temperature increases. As a consequence, the current, as well as the power, supplied to the LED load increases with a rise in temperature. As the current increases, this, in turn, may create more heat, which results in even higher current delivered to the load. This, in effect, may result in a thermal runaway, which may eventually lead to a burn out of the LED-based load.
In the case of a constant output current ballast, it employs output current feedback to ensure that the current through the LED-based load is substantially constant. However, as discussed above, the junction voltage of LED devices decreases as environment temperature increases. This has the consequence of the output voltage, as well as the power, decreasing with a rise in temperature. In this case, the LED light output will decrease with rising temperature, which may be undesirable for lots of applications.
Another issue with constant output voltage and current ballasts is that they do not work well with phase control dimming circuits. A phase control dimming circuit controls the amount of power delivered to a luminous load by suppressing or cutting off a portion of the rectified input voltage. Accordingly, as the dimmer is controlled to reduce the brightness of the luminous load, the constant output voltage or current ballast would sense the output voltage or current reduction due to the dimmer, and try to increase the same to maintain the same output voltage or current. As a result, the brightness of the luminous load remains fairly the same, even though the dimmer is attempting to reduce the brightness. This renders the dimmer ineffective.